1. Field of the Invention
The present invention relates to a novel fluoran compound which is useful as a chromogenic compound in a heat-sensitive recording materials, to a process for the preparation of the compound and heat-sensitive recording materials comprising said compound.
2. Description of the Prior Art
Heat-sensitive recording systemsutilizing a color reaction between a colorless or pale colored electron donative compound (chromogenic compound) and an organic or inorganic electron acceptor (developer) have been widely popularized.
In the recording system, fluoran compounds have widely been used as the chromogenic compunds.
Many fluoran compounds are known in the prior art, for example those having the formulas (A), (B) and (C). ##STR4## Japanese Patent Publication SHO 48-43296 (1973) ##STR5## Japanese Patent Publication SHO 51-23204 (1976) ##STR6## Japanese Patent Laid-Open Publication SHO 60-35053 (1985).
However, when used in a heat-sensitive recording material, the compound of the formula (A) has the defect that the compound itself colors gray to dark gray when mixed with a developer such as bisphenol A and imparts a a gray to dark gray (soiled) color to paper when the mixture is applied to a paper.
The compound of the formula (B) or the formula (C) provides a paper having relatively high whiteness immediately thereto but, the coated paper has a disadvantage of becoming gray or brown under the influence of moisture, light or heat.
The heat-sensitive recording material has recently been used indoors as in the case of facsimile paper and additionally in the open air under more severe conditions as in the case of, for example, prepaid cards such as telephone cards. Consequently, the heat-sensitive recording materials are required to further enhance preserving stability.